The mechanism of transition (reaction coordinate) during an activated process is best described in terms of the isocommittor surfaces. These surfaces can be used to identify effective transition tubes inside which the reactive trajectories involved in the activated process stay confined. It is shown that the isocommittor surfaces can be identified directly, i.e., without ever sampling actual reactive trajectories, and some procedures to turn this observation into practical algorithms such as the finite temperature string method are discussed. (c) 2005 Elsevier B.V. All rights reserved.